However, the Conservatives have claimed that the Labour leader would need to apply his mansion tax to all homes worth over £415,000 in order to be able to afford to reintroduce the 10p tax band.

According to Tory research disclosed yesterday, Labour would have to impose the homes tax on more than two million families across the country to raise the £7.3 billion required to restore the 10p tax rate, which was abolished by Gordon Brown.

Grant Shapps, the Conservative Party Chairman, described Labour’s property levy proposals as a “homes tax con”.

“Labour’s mansion tax is quickly unravelling into a tax on millions of hardworking people who are not living in mansions at all,” he said. “People will wake up and suddenly find themselves defined as living in a mansion.”

The Lib Dems have been arguing in the Coalition for a mansion tax to be introduced on the most valuable homes but George Osborne, the Chancellor, has repeatedly blocked the move.

They will table an amendment allowing MPs to express support for the principle of the mansion tax without having to vote with the Opposition for the proposal.

“This amendment allows Liberal Democrats in Parliament to back our long-held policy of the mansion tax,” Mr Cable said. “We created it and will continue to champion it.”

He added: “In Government, Labour rubbished the Liberal Democrat policy of a mansion tax. In Opposition, they have simply copied it exactly in an attempt to fill in their blank piece of paper where original policies should be.”

Labour reacted angrily to the Lib Dem decision, saying it would be "astonishing" if they failed to back their own "long-held policy".

Chris Leslie, the shadow Treasury minister, said: "The Liberal Democrats have a simple choice – either they back the policy they set out in their manifesto and which Nick Clegg made the centerpiece of the Eastleigh by-election campaign or they don’t."